Make graph Clone.

src/lib.rs

@@ -21,6 +21,7 @@ pub use node::*;
 
 /// A generic graph type holding values connected to other values.
 /// Values can be added to the graph, but not removed.
+#[derive(Clone)]
 pub struct Graph<NodeValueT: NodeValue> {
     nodes: Vec<Node<NodeValueT>>,
 }